Maintenance of fracture toughness of welded joints of X80 large diameter thick-walled pipes
Keywords:
welded joint, heat affected zone, cooling rate, toughness, microstructure, bainiteAbstract
From practice and mechanical tests experience it is well known, that the metal of the heat affected zone (HAZ) of welded joints is subjected to damaging much more than the base metal. In the course of numerous tests on fracture toughness it was found that the coarse grain zone (CGHAZ) and the fusion line (CGHAZ) have the lowest welding toughness. Such reduction in toughness of welded joints of submerged arc welded large diameter pipes leads to the production rejection and serious economic consequences.
In order to identify the reasons of such metal behavior in HAZ welding, modeling of thermal cycles was simulated on samples from steels X80 using Gleeble 3800. During the work the relationship between the microstructure formed in the CGHAZ and toughness of welded joints of high-strength large diameter pipes was established. The most unfavorable type of CGHAZ microstructure is granular bainite. Negative effect of vanadium and silicon on toughness of welded joints of high-strength large diameter pipes was found. Also reasons of toughness reduction in weld metal were found. Great reduction of toughness in weld metal occurs when ferrite precipitates along primary dendrites boundaries.
According to the results of studies the chemical composition of base metal was designed and the method of increasing toughness of weld metal by the addition of titanium and boron into the weld puddle was recommended. Recommendations were used in the production of the experimental lot of pipes for the project “The Power of Siberia”. The results of mechanical tests showed high toughness of welded joints.
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